Electrochemical transducers are generally subdivided into the three groups of potentiometric, conductometric and amperometric. In the case of potentiometric transducers, the potential is measured with respect to a reference electrode. Ion-selective sensors operate on this basis, and the electrode is in this case coated with an ion-selective membrane. The potential on the electrode is then a measure of the concentration of the corresponding ions. A potentiometric pCO2 sensor can thus also be produced by way of a gas-permeable membrane.
In the case of amperometric transducers, in contrast, a voltage difference is produced between two electrodes, in the case of which the substance to be detected is converted. The currents which flow during the reduction or oxidation process result in the measurement signal. These are widely used as oxygen sensors or biochemical sensors. In the case of a Clark-analogous oxygen sensor, a gas-permeable membrane is applied to the amperometric sensor.
In the case of biochemical sensors, molecular identification systems, such as haptens, antigens or antibodies are placed on or in the vicinity of the electrodes. The target molecule binds thereto and is provided either directly or via intermediate steps with an enzyme label. If the corresponding enzyme substrate is now added, the enzyme releases a substance which can be detected. This is done either optically or electrochemically. This is the so-called ELISA test (Enzyme Linked Immuno Sorbent Assay). DNA analysis methods can also be carried out in a similar way.
The transducers which are used for electrochemical detection must include electrodes with which electrical contact is made individually. During use of potentiometric transducers, the resultant equilibrium potential with respect to a reference electrode must be able to be measured. In the case of amperometric and conductometric transducers, it must be possible to potentiostat the electrodes, and it must be possible to detect the current flow through the electrodes individually.
One example of planar ion-selective sensors is described in E. Jacobs et al, “Analytical Evaluation of i-STAT Portable Clinical Analyzer and Use by Nonlaboratory Health-Care Professionals”, Clinical Chemistry, 39, 1069 et seq. (1993). This is a silicon substrate with thin-film electrodes and ion-selective membranes. The sensor electrodes and contacts are in this case located on the same side of the silicon substrate. In order thus to separate the contact surfaces and the flow cell for the analyte, the substrate must be considerably larger than the area which is actually required by the sensors.
Various biochips are likewise manufactured using silicon technology, and are described R. Thewes et al, “Sensor Arrays for Fully Electronic DNA Detection on CMOS”, ISSCC Digest of Tech. Papers, 2002, 350 et seq. This has the advantage of the integration of CMOS circuit technology, signal processing (multiplexing) and analog/digital conversion in the sensor platform itself. A large number of sensors can thus be provided in a very small area. One disadvantage relates to the costs for production of a chip such as this and the complex handling (contact-making). The costs per individual sensor are thus high for so-called low-density arrays with fewer than 100 sensors per square centimeter.
Theoretically, it is possible to use polymer mounts with electrodes fitted to the polymer mounts, as an alternative. These can be vapor-deposited or printed on. This method makes it possible to produce individual sensors, for example glucose sensors, at low cost [WO2002/02796-A2]. However, it is not very suitable for arrays since the conductor track structures are coarse, so that the number of electrical contacts is greatly restricted.
Printed circuit board technology is used in the already known eSensor™ from the Motorola Company in order to produce a “low-density” DNA detection system. In this case, both the sensor surfaces and the conductor tracks and contacts are formed on the metallization layer. The product is a rigid printed circuit board with sensors and contacts on the same side. Rear-face contacts can be provided by through-plating. This technique can, however, be implemented only at high cost for large-scale manufacture.
Furthermore, by way of example, so-called microelectrode arrays are known from EP 0 504 196 B1 and DE 297 17 809 U1, in which the sensor cavities have as small an area as possible. DE 199 16 921 A1 discloses a method for production of arrays which are arranged in pairs and are composed of microelectrodes, in which the mount is either silicon or plastic. The aim in this case is to be able to drive the individual electrodes separately. DNA analysis is quoted in particular as an application.
Finally, WO 2004/001404 A1 discloses an array of microelectrodes in which the structure can be varied. The array mounts are in this case glass and/or Captan films, with a single reference-ground electrode being used. Finally, DE 199 29 264 A1 discloses a universal transducer for chemosensors and biosensors, in which a multilayer system is provided with isolating layers and electrode layers, which are used as working, reference-ground and counterelectrodes. The large number of known transducer arrays therefore place particular emphasis on specific microelectrodes, with contact always being made from above.